1. Field of the Invention
The present invention is directed to equipment for drilling in subterranean formations. More particularly, the present invention is directed to a device which is incorporated in the drill string and which facilitates release of stuck portions of the drill string.
2. Brief Description of the Prior Art
Equipment used for lowering a drill bit into subterranean formations usually includes a string of drill pipes and a bottom hole assembly containing a plurality of drill collars. The drill collars are pipes which have larger diameter, thicker wall and therefore heavier weight than the drill pipes of the upper portions of the drill string. The drill collars are usually necessary for providing sufficient weight for the operation of the drill bit which is at the bottom of the assembly.
The drill string is rotated from the surface by a rotary table. In conventional drilling operations, the rotary table rotates the drill string and provides the power for driving the drill bit into the formation. In other drilling operations, the power to drive the drill bit is derived from a downhole motor.
The downhole motor is usually driven by the pressure of drilling mud which is continuously pumped downhole in the interior of the drill string and which rises to the surface between the walls of the bore hole and the exterior walls of the drill string.
As is well known, portions of the drill string, particularly the lower portions, occasionally become strongly attached, "differentially stuck", to walls of the bore hole. The reason for this is that the external diameter of the drill collars is relatively close to the diameter of the bore hole, and that the drilling mud is circulated in the bore hole under high pressure. This pressure often exceeds several thousands of pounds per square inch (psi). When for some reason, pressurized drilling mud is excluded from between the wall of the drill collar and the subterranean formation then the hydrostatic pressure prevailing in the hole presses the drill collar against the formation with tremendous force. Most frequently such "differential sticking" of a portion of the drill collar to the formation occurs because of accumulation of mud or debris between the drill collar and the formation.
Regardless of the original reason for sticking of the drill collar (or drill pipes) in the bore hole, the phenomenon represents a serious and expensive-to-solve problem for the drilling industry. As is well known, when the drill collar is stuck, the actual drilling operation halts. In accordance with usual practice in the art, the workmen at the drilling rig first try to free the stuck drill collars. When the attempts to free the stuck drill collars fail, then the stuck portion is usually severed from the portion of the drill string which is located above it. The stuck portion of the drill string is either abandoned in the bore hole or is retrieved ("fished-out") from the hole by using various retrieval techniques and devices.
Unfortunately from the view point of the drilling industry, accumulation of solid mud or debris between the walls of a drill collar and the formation is usually accelerated whenever rotation or movement of the drill collars is halted. Thus, unless a differentially stuck drill collar is freed promptly, larger and larger portions become hydrostatically pressed against the formation and freeing the assembly becomes progressively more difficult.
Even though the prior art has been well aware of the abovesummarized problem, it has been, by-and-large, unable to provide an effective solution. Severing the free portion of the drill string from the stuck portion and thereafter abandoning the stuck portion (free point and back-off) is not considered an effective solution in this regard. Other attempts to solve the problem include devices which are located at the top of the drill collars and provide repeated, large, axially directed jolts to the drill string. Many times, however, these devices are unable to free the stuck drill string. For these reasons the present invention fills a great need in the prior art.